How Drying Preserves Food - University Of Georgia
Food drying is one of the oldest methods of preserving food for later use. It can either be analternative to canning or freezing, or compliment these methods. Drying foods is simple, safeand easy to learn. With modern food dehydrators, fruit leathers, banana chips and beef jerkycan all be dried year round at home.How Drying Preserves FoodDrying removes the moisture from the food so bacteria, yeast and mold cannot grow and spoilthe food. Drying also slows down the action of enzymes (naturally occurring substances whichcause foods to ripen), but does not inactivate them.Because drying removes moisture, the food becomes smaller and lighter in weight. When thefood is ready for use, the water is added back, and the food returns to its original shape.Foods can be dried in the sun, in an oven or in a food dehydrator by using the right combinationof warm temperatures, low humidity and air current.In drying, warm temperatures cause the moisture to evaporate. Low humidity allows moisture tomove quickly from the food to the air. Air current speeds up drying by moving the surroundingmoist air away from the food.Drying Foods Out-of-DoorsSun DryingThe high sugar and acid content of fruits make them safe to dry in the sun. Vegetables andmeats are not recommended for sun drying. Vegetables are low in sugar and acid. This increasesthe risks for food spoilage. Meats are high in protein making them ideal for microbial growthwhen heat and humidity cannot be controlled.To dry in the sun, hot, dry, breezy days are best. A minimum temperature of 86 F is neededwith higher temperatures being better. It takes several days to dry foods out-of-doors. Becausethe weather is uncontrollable, sun drying can be risky.Also, the high humidity in the South is a problem. A humidity below 60 percent is best forsun drying. Often these ideal conditions are not available when fruit ripens.Fruits dried in the sun are placed on trays made of screen or wooden dowels. Screens needto be safe for contact with food. The best screens are stainless steel, teflon coated fiberglassor plastic. Avoid screens made from "hardware cloth." This is galvanized metal cloth thatis coated with cadmium or zinc. These materialscan oxidize, leaving harmful residues on the food.Also avoid copper and aluminum screening.Copper destroys vitamin C and increases oxidation.Aluminum tends to discolor and corrode.Outdoor Drying Rack
1. Freezer Method - Seal the food in freezer-typeplastic bags. Place the bags in a freezer set at 0 For below and leave them at least 48 hours.Most woods are fine for making trays. However,do not use green wood, pine, cedar, oak or redwood.These woods warp, stain the food or cause off-flavorsin the food.2. Oven Method - Place the food in a single layeron a tray or in a shallow pan. Place in an ovenpreheated to 160 F for 30 minutes.Place trays on blocks to allow for better air movementaround the food. Because the ground may be moist,it is best to place the racks or screens on a concretedriveway or if possible over a sheet of aluminum or tin.The reflection of the sun on the metal increases thedrying temperature. Cover the trays with cheeseclothto help protect the fruit from birds or insects. Fruitsdried in the sun must be covered or brought undershelter at night. The cool night air condenses and couldadd moisture back to the food, thus slowing down thedrying process.After either of these treatments the dried fruit is readyto be conditioned and stored.Drying Foods IndoorsMost foods can be dried indoors using modern dehydrators, convection ovens or conventional ovens. Microwaveovens are recommended only for drying herbs, becausethere is no way to create airflow in them.Solar DryingRecent efforts to improve on sun drying have led to solardrying. Solar drying also uses the sun as the heat source.A foil surface inside the dehydrator helps to increase thetemperature. Ventilation speeds up the drying time.Shorter drying times reduce the risks of food spoilage ormold growth.Food DehydratorsA food dehydrator is a small electrical appliance fordrying food indoors. A food dehydrator has an electricelement for heat and a fan and vents for air circulation.Dehydrators are efficiently designed to dry foods quicklyat 140 F.Food dehydrators are a relatively new item and areavailable from department stores, mail-order catalogs,natural food stores, seed catalogs and garden supplystores. Costs vary from 40 to 350 or above dependingon features. Some models are expandable and additionaltrays can be purchased later. Twelve square feet of dryingspace dries about a half-bushel of produce.airflowplastic sheetingOven DryingEveryone who has an oven has a dehydrator. Bycombining the factors of heat, low humidity and airflow, an oven can be used as a dehydrator.foilsurfaceAn oven is ideal for occasional drying of meat jerkies,fruit leathers, banana chips or for preserving excessproduce like celery or mushrooms. Because the oven isneeded for every day cooking, it may not be satisfactoryfor preserving abundant garden produce.Homemade Solar DryerOven drying is slower than dehydrators because it doesnot have a built-in fan for the air movement. (However,some convection ovens do have a fan). It takes abouttwo times longer to dry food in an oven than it doesin a dehydrator. Thus, the oven is not as efficient asa dehydrator and uses more energy.PasteurizationSun or solar dried fruits and vine dried beans needtreatment to kill any insect and their eggs that mightbe on the food. Unless destroyed, the insects will eatthe dried food. There are two recommendedpasteurization methods:2
To Use Your Oven - First, check the dial and see if itcan register as low as 140 F. If your oven does not gothis low, then your food will cook instead of dry. Usea thermometer to check the temperature at the "warm"setting.Pretreating the FruitFor air circulation, leave the oven door proppedopen two to six inches. Circulation can be improvedby placing a fan outside the oven near the door.CAUTION: This is not a safe practice for a homewith small children.For long-term storage of dried fruit, sulfuring or usinga sulfite dip are the best pretreatments. However, sulfitesfound in the food after either of these treatments havebeen found to cause asthmatic reactions in a smallportion of the asthmatic population. Thus, somepeople may want to use the alternative shorter-termpretreatments. If home dried foods are eaten withina short time, there may be little difference in thelong- and short-term pretreatments.Pretreatments prevent fruits from darkening. Manylight-colored fruits, such as apples, darken rapidly whencut and exposed to air. If not pretreated, these fruitswill continue to darken after they have dried.Because the door is left open, the temperature will vary.An oven thermometer placed near the food gives anaccurate reading. Adjust the temperature dial to achievethe needed 140 F.Sulfuring - Sulfuring is an old method of pretreatingfruits. Sublimed sulfur is ignited and burned in anenclosed box with the fruit. The sulfur fumes penetratethe fruit and act as a pretreatment by retarding spoilageand darkening of the fruit. Fruits must be sulfuredout-of-doors where there is adequate air circulation.(For more information contact your county Extensionoffice.)Drying trays should be narrow enough to clear the sidesof the oven and should be 3 to 4 inches shorter than theoven from front to back. Cake cooling racks placed ontop of cookie sheets work well for some foods. The ovenracks, holding the trays, should be two to three inchesapart for air circulation.Sulfite Dip - Sulfite dips can achieve the samelong-term anti-darkening effect as sulfuring, but morequickly and easily. Either sodium bisulfite, sodiumsulfite or sodium meta-bisulfite that are USP (foodgrade) or Reagant grade (pure) can be used. To locatethese, check with your local drugstores or hobby shops,where wine-making supplies are sold.DRYING FRUITSDried fruits are unique, tasty and nutritious. Begin bywashing the fruit and coring it, if needed. For drying,fruits can be cut in half or sliced. Some can be leftwhole. See the table “Drying Fruits at Home” later inthis publication for specific directions for preparingeach fruit.Directions for Use - Dissolve 3 4 to 1 1 2 teaspoonssodium bisulfite per quart of water. (If using sodiumsulfite, use 1 1 2 to 3 teaspoons. If using sodiummetabisulfite, use 1 to 2 tablespoons.) Place theprepared fruit in the mixture and soak 5 minutes forslices, 15 minutes for halves. Remove fruit, rinse lightlyunder cold water and place on drying trays. Sulfitedfoods can be dried indoors or outdoors. (This solutioncan be used only once. Make a new one for the nextbatch.)Thin, uniform, peeled slices dry the fastest. The peelcan be left on the fruit, but unpeeled fruit takes thelonger to dry. Apples can be cored and sliced in rings,wedges, or chips. Bananas can be sliced in coins, chipsor sticks.Fruits dried whole take the longest to dry. Beforedrying, skins need to be "checked" or cracked to speeddrying. To "check" the fruit place it in boiling waterand then in cold water.Ascorbic Acid - Ascorbic acid (vitamin C) mixed withwater is a safe way to prevent fruit browning. However,its protection does not last as long as sulfuring orsulfiting. Ascorbic acid is available in the powdered ortablet form, from drugstores or grocery stores. Oneteaspoon of powdered ascorbic acid is equal to 3000 mgBecause fruits contain sugar and are sticky, spray thedrying trays with nonstick cooking spray before placingthe fruit on the trays. After the fruit dries for one to twohours, lift each piece gently with a spatula and turn.3
boil. Add 1 pound of prepared fruit and simmer 10minutes. Remove heat and let fruit stand in hot syrup for30 minutes. Lift fruit out of syrup, rinse lightly in coldwater, drain on paper toweling and place on dryer trays.of ascorbic acid in tablet form. (If you buy 500 mgtablets, this would be six tablets).Directions for Use - Mix 1 teaspoon of powderedascorbic acid (or 3000 mg of ascorbic acid tablets,crushed) in 2 cups water. Place the fruit in the solutionfor 3 to 5 minutes. Remove fruit, drain well and place ondryer trays. After this solution is used twice, add moreacid.Steam Blanching - Steam blanching also helps retaincolor and slow oxidation. However, the flavor and textureof the fruit is changed.Directions - Place several inches of water in a largesaucepot with a tight fitting lid. Heat to boiling. Placefruit not more than 2 inches deep, in a steamer pan orwire basket over boiling water. Cover tightly with lidand begin timing immediately. See below for blanchingtimes. Check for even blanching half way through theblanching time. Some fruit may need to be stirred.When done, remove excess moisture using paper towelsand place on dryer trays.Ascorbic Acid Mixtures - Ascorbic acid mixtures are amixture of ascorbic acid and sugar sold for use on freshfruits and in canning or freezing. It is more expensiveand not as effective as using pure ascorbic acid.Directions for Use - Mix 1 1 2 tablespoons of ascorbic acidmixture with one quart of water. Place the fruit in themixture and soak 3 to 5 minutes. Drain the fruit welland place on dryer trays. After this solution is used twice,add more ascorbic acid mixture.Drying the Prepared FruitWhichever drying method you choose-sun drying, solardrying, oven drying or dehydrator drying-be sure toplace the fruit in a single layer on the drying trays. Thepieces should not touch or overlap. Follow the directionsfor the drying method you choose and dry until thefood tests dry. Approximate drying times are givenbelow. Food dries much faster at the end of the dryingperiod, so watch it closely.Fruit Juice Dip - A fruit juice that is high in vitamin Ccan also be used as a pretreatment, though it is not aseffective as pure ascorbic acid. Juices high in vitamin Cinclude orange, lemon, pineapple, grape and cranberry.Each juice adds its own color and flavor to the fruit.Directions for Use - Place enough juice to cover fruit in abowl. Add cut fruit. Soak 3 to 5 minutes, remove fruit,drain well and place on dryer trays. This solution maybe used twice, before being replaced. (The used juicecan be consumed.)Determining Dryness of FruitsSince dried fruits are generally eaten without beingrehydrated, they should not be dehydrated to the pointof brittleness. Most fruits should have about 20 percentmoisture content when dried.Honey Dip - Many store-bought dried fruits have beendipped in a honey solution. A similar dip can be made athome. Honey dipped fruit is much higher in calories.To test for dryness, cut several cooled pieces in half.There should be no visible moisture and you should notbe able to squeeze any moisture from the fruit. Somefruits may remain pliable, but are not sticky or tacky.If a piece is folded in half, it should not stick to itself.Berries should be dried until they rattle when shaken.Directions for Use - Mix 1 2 cup sugar with 1 1 2 cupsboiling water. Cool to lukewarm and add 1/2 cup honey.Place fruit in dip and soak 3 to 5 minutes. Remove,drain well and place on dryer trays.After drying, cool fruit 30 to 60 minutes beforepackaging. Packaging food warm can lead to sweatingand moisture buildup. However, excessive delays inpackaging could allow moisture to re-enter food.Remember, if you have dried fruit in the sun, it mustbe pasteurized before it is packaged.Syrup Blanching - Blanching fruit in syrup helps itretain color fairly well during drying and storage. Theresulting product is similar to candied fruit. Fruits thatcan be syrup blanched include apples, apricots, figs,nectarines, peaches, pears, plums and prunes.Directions for Use - Combine 1 cup sugar, 1 cup lightcorn syrup and 2 cups water in a saucepot. Bring to a4
water. Cover and blanch according to directions. Begintiming when water returns to boiling. If it takes longerthan one minute for the water to come back to boiling,too many vegetables were added. Reduce the amountin the next batch.Conditioning FruitsWhen dried fruit is taken from the dehydrator or oven,the remaining moisture may not be distributed equallyamong the pieces because of their size or their location inthe dehydrator. Conditioning is a process used to equalize the moisture and reduce the risk of mold growth.Steam Blanching - Use a deep pot with a tight fittinglid and a wire basket, colander or sieve placed so thesteam will circulate freely around the vegetables. Addwater to the pot and bring to a rolling boil. Place thevegetables loosely in the basket no more than 2 inchesdeep. Place the basket of vegetables in the pot, makingsure the water does not come in contact with thevegetables. Cover and steam according to the directions.To condition the fruit, take the dried fruit that hascooled and pack it loosely in plastic or glass jars. Seal thecontainers and let them stand for seven to ten days. Theexcess moisture in some pieces will be absorbed by thedrier pieces. Shake the jars daily to separate the piecesand check the moisture condensation. If condensationdevelops in the jar, return the fruit to the dehydrator formore drying. After conditioning, package and store thefruit.Cooling and Drying the Prepared VegetablesAfter blanching, dip the vegetables briefly in cold water.When they feel only slightly hot to the touch, drain thevegetables by pouring them directly onto the drying trayheld over the sink. Wipe the excess water from underneath the tray and arrange the vegetables in a single layer.Then place the tray immediately in the dehydrator oroven. The heat left in the vegetables from blanching willcause the drying process to begin more quickly. Watchthe vegetables closely at the end of the drying period.They dry much more quickly at the end and couldscorch.DRYING VEGETABLESVegetables can also be preserved by drying. Because theycontain less acid than fruits, vegetables are dried untilthey are brittle. At this stage, only 10% moisture remainsand no microorganism can grow.Preparing VegetablesTo prepare vegetables for drying, wash in cool water toremove soil and chemical residues. Trim, peel, cut, sliceor shred vegetables according to the directions for eachvegetable in the chart below. Remove any fibrous orwoody portions and core when necessary, removing alldecayed and bruised areas. Keep pieces uniform in sizeso they will dry at the same rate. A food slicer or foodprocessor can be used. Prepare only as many as can bedried at one time.Determining Dryness of VegetablesVegetables should be dried until they are brittle or"crisp." Some vegetables would actually shatter if hitwith a hammer. At this stage, they should containabout 10 percent moisture. Because they are so dry,they do not need conditioning like fruits.Pretreating VegetablesDRYING FRUIT LEATHERBlanching is a necessary step in preparing vegetables fordrying. By definition, blanching is the process of heatingvegetables to a temperature high enough to destroyenzymes present in tissue. Blanching stops the enzymeaction which could cause loss of color and flavor duringdrying and storage. It also shortens the drying andrehydration time by relaxing the tissue walls so moisturecan escape and later re-enter more rapidly.Fruit leather is a tasty, chewy, dried fruit product. Fruitleathers are made by pouring puréed fruit onto a flatsurface for drying. When dried, the fruit is pulled fromthe surface and rolled. It gets the name "leather" fromthe fact that when puréed fruit is dried, it is shiny andhas the texture of leather.Leather From Fresh FruitVegetables can be water blanched or steam blanched.Water blanching usually results in a greater loss ofnutrients, but it takes less time than steam blanching.* Select ripe or slightly overripe fruit.* Wash fresh fruit or berries in cool water. Removepeel, seeds and stem.* Cut fruit into chunks. Use 2 cups of fruit for each13" x 15" inch fruit leather. Purée fruit until smooth.* Add 2 teaspoons of lemon juice or 1/8 teaspoonWater Blanching - Fill a large pot 2/3 full of water,cover and bring to a rolling boil. Place the vegetables ina wire basket or a colander and submerge them in the5
rodent proof. Fruit that has been sulfured or sulfitedshould not touch metal. Place the fruit in a plastic bagbefore storing it in a metal can.ascorbic acid (375 mg) for each 2 cups of lightcolored fruit to prevent darkening.* Optional: To sweeten, add corn syrup, honey or sugar.Corn syrup or honey is best for longer storage becauseit prevents crystals. Sugar is fine for immediate useor short storage. Use 1 4 to 1 2 cup sugar, corn syrupor honey for each 2 cups of fruit. Saccharin-basedsweeteners could also be used to reduce tartnesswithout adding calories. Aspartame sweeteners maylose sweetness during drying.Dried food should be stored in a cool, dry, dark place.Most dried fruits can be stored for 1 year at 60 F, 6months at 80 F. Dried vegetables have about half theshelf-life of fruits. Fruit leathers should keep for up to1 month at room temperature. To store any driedproduct longer, place it in the freezer.Leathers From Canned or Frozen FruitsUSING DRIED FOODS* Home preserved or store bought canned or frozenfruit can be used.* Drain fruit, save liquid.* Use 1 pint of fruit for each 13" x 15" leather.* Purée fruit until smooth. If thick, add liquid.* Add 2 teaspoons of lemon juice or 1/8 teaspoonascorbic acid (375 mg) for each 2 cups of lightcolored fruit to prevent darkening.* If desired, sweeten as directed above for leathersfrom fresh fruit.* Applesauce can be dried alone or added to any freshfruit purée as an extender. It decreases tartness andmakes leather smoother and more pliable.Dried fruits can be eaten as is or reconstituted. Driedvegetables must be reconstituted. Once reconstituted,dried fruits or vegetables are treated as fresh. Fruitleathers and meat jerky are eaten as is.To reconstitute dried fruits or vegetables, add water tothe fruit or vegetable and soak until the desired volumei
Solar Drying Recent efforts to improve on sun drying have led to solar drying. Solar drying also uses the sun as the heat source. A foil surface inside the dehydrator helps to increase the temperature. Ventilation speeds up the drying time. Shorter drying times reduce the risks of food spoilage or mold growth. Pasteurization
Air- and Shed-drying Lumber J.E. Reeb and T.D. Brown EM 8612-E Reprinted October 2007 The process of drying wood is the same for air- and kiln-drying, but in kiln-drying you have much greater control over air velocity, temperature, and humidity. Controls are much less when air-drying lumber.
Me- EMC, %db . Mo – Initial moisture content, %db . K – drying constant . θ - time, hour . Deep bed drying All grains are not fully exposed to the same condition of drying air . Grain fed at the top & move downward in a zig-zag path where it encounters a cross flow of hot air
National Center for Home Food Preservation Judging Home Preserved Foods - p. 34 Judging Jams, Jellies and Preserves Headspace for jams, jellies, other sweet preserves and butters should be ¼ inch. Canned jams, jellies and preserves should meet all of the general characteristics for container, pack, appearance of contents and attractiveness.
1 Drying of Food at Home Preserving food by drying is the oldest method of food preservation. Sun drying of fruits and vegetables was practiced b
2 · FREEZE DRYING In our RAY freeze dryers, frozen products are dried at temperatures below -18 C. No thawing of the product takes place and its quality is preserved. Pressure Torr Water phase diagram Temperature 1000 760 100 10 4.6 1 Hot air drying Vacuum drying Product (Room Temperature at sea level pressure) Freeze drying Triple .
Sun-Drying Because sun-drying takes more time, pretreating the produce by blanching or another method is much more important. The ideal temperature is about 100 F with low humidity. If you are blessed with a climate like that, do try sun-drying. In other climates, use caution
viz., moisture content, psychrometric properties, water activity of food are described in this chapter for thorough understanding of detailed drying aspects and applications discussed in subsequent chapters in this book. 2 Principle of Drying Drying process is governed by principle of heat and mass transfer. The moist
RTS performs tree risk assessment in accordance with ANSI A300 (Part 9) - Tree Risk Assessment. Not only because we must as ISA Certified Arborists who are Tree Risk Assessment Qualified (TRAQ), but also because it ensures consistency by providing a standardized and systematic process for assessing tree risk. Risk assessment via TRAQ methodology takes one of three levels, depending on the .
